An enamel-insulated coil of a motor and/or a dynamo (generator) mounted in a vehicle such as a car or a train or in an industrial machine can be affected by humidity, water, dust, and the like in the environment of use of the coil. For example, when such substances in the environment penetrate and accumulate between coil wires, the electrical insulating properties of the enamel coating are deteriorated. This causes short-circuits between neighboring wires and consequently a deterioration of a motor or a reduction in the electromotive force of a generator.
Friction occasionally develops in an insulated coil between neighboring coil wires and/or between coil wires and the core of the coil due to vibrations of a motor or of a rotor of a generator which runs at high speed, or due to vibrations or impacts from equipment such as a car on which a motor or generator is mounted. There is the risk of the coil wires being severed due to this friction, which produces fatal problems for a motor or generator.
In order to prevent such deterioration in insulating properties and to protect against damage to or severing of an insulated coil, an insulated coil is impregnated with an epoxy resin and hardened so that adjoining coil wires adhere to each other and to the core of the coil.
Recently, the properties that the above-described epoxy resin composition needs to meet have become more severe especially for use in automobiles. Because of the trend that automobiles hybridizes in their drive mechanisms, motors for automobiles, especially a drive motor-generator sets are being decreased in size and upgraded in performance, and as a result, the winding density of insulated coils and/or the rotational speed becomes higher. Therefore, heat generation by motors, and especially by drive motor-generator set, increases. Therefore, the thermal stability of conventional epoxy resins for impregnation and fixation has become insufficient. In addition, because the reliability of the drive motor-generator set of a hybrid car needs to be as high as that of a conventional combustion engine, requirements concerning the thermal stability over time of a hardened resin and its stability in an impregnation and fixation process have become extremely severe. Furthermore, there is a persistent, strong demand for increase in the productivity of the impregnation and fixation process.
An example of a conventional epoxy resin composition is disclosed in Patent Document 1. The composition consists of an epoxy compound in liquid form selected from the group consisting of bisphenol F type epoxy resins and cycloaliphatic type epoxy resins; a bismaleimide; an acid anhydride in liquid form; and an amine series latent hardening accelerator. Although this composition has superior storage stability because it is a one-part agent, its shear bond strength after storage in a hot environment for a long time cannot sufficiently meet the present-day demand mentioned above. Even if its strength is at a certain level just after hardening, the strength deteriorates in a short period during storage in a hot environment, which increases risks such as disconnection of the coil wire.
In order to achieve the high thermal stability mentioned above, various improvements have been made to epoxy resin compositions. For instance, it has been proposed to add a bulking agent such as inorganic microparticles to the composition.
As a specific example, an epoxy resin composition for insulation of a coil in electric equipment and the like is disclosed in Patent Document 2. The composition consists of an epoxy compound, bismaleimide, a hardener of an acid anhydride, a hardening accelerator containing imidazole compounds, and a silica micropowder having a prescribed particle size distribution. In this composition, the particle size is strictly specified in order to avoid a decrease in the operability of the process, i.e., a deterioration with age in its viscosity, which is caused by the addition of the micropowder to improve the mechanical properties.
However, an improvement such as the addition of bulking agents eventually increases the number of parameters being controlled in the manufacturing process and increases the difficulty of process control. In addition, use of imidazole compounds in this composition increases the activity of the resin composition. Therefore, the compound cannot substantially be used in a one-part form in order to ensure storage stability and hence has to be used in a two-part form in many cases. Consequently, use of this compound obviously complicates the impregnation and fixation process, which runs counter to the trend of improving productivity. Furthermore, because the trend of decreasing the size of insulated coils is accompanied by an increase in the winding density of coil wires, a resin containing a micropowder often cannot reach between coil wires. In such a case, there is a concern that the composition cannot perform the basic task of bonding the coil wires. As mentioned above, because bulking agents which are added to improve thermal stability destabilize the impregnation and fixation process, it is difficult to apply the above-described composition to automobiles and the like, in which safety is important.
As another example, it was proposed in Patent Document 3 to specify the water content in a composition just after preparation. Namely, the one-part composition disclosed in Patent Document 3 consists of an epoxy compound having a plurality of epoxy groups in each molecule, a polycarboxylic acid anhydride in liquid form, and a hardener, wherein the water content just after preparation of the composition is less than 800 ppm by weight. This composition also increases the number of control parameters and causes additional problems in manufacturing. Particularly, because an additional control parameter is the water content, which is easily affected by the humidity of the environmental, a proper storage environment has to be developed for each component, and the mixing ratio of each component has to be modified according to the humidity where the blending process is performed.
Patent Document 1: JP63-251417A
Patent Document 2: JP07-128575A
Patent Document 3: JP2002-145996A